I believe it’s almost impossible to judge that because drivers’ performances can change a lot from one season to another. For instance, most fans would probably have put Massa in TOP 5 if this question had been asked in 2008 or even 2009. And how often we would have seen Button in TOP 5 in 2008?

But if I really had to choose the best one from the current grid then I would say it’s Alonso. I think he mostly handles pressure better than Hamilton or Vettel.

@Girts : Vettel proved he can handle pressure at the end of last year when he came back from zero to hero, proving great mental strenght. On the other hand, Alonso lost to rookie Hamilton when he found himself under pressure, and I have to also that Fernando was always protected by his team bosses, first Briatore at Renault, where his supremacy in the team was never in doubt, now at Ferrari versus Massa.
Don’t get me wrong, I think very high about Alonso, but he had his bad moments, as Vettel, Button and Hamilton did.

@Cristian I think it’s rather Alonso’s qualities and personality that have ensured him the dominance of his team mates, not special attitude from his bosses. I also believe that Alonso actually coped with the 2007 situation very well, he still showed many fantastic performances as the atmosphere around him had got dreadful. But that’s just my impression.